Argon is a highly inert element over a very wide range of conditions, both at cryogenic and very high temperatures. It is used in steel-making, light bulbs, electronics, welding and gas chromatography. The major source of argon is that found in the air and it is typically produced therefrom using cryogenic air separation units. The world demand for argon is increasing and thus it is essential to develop an efficient process which can produce argon at high recoveries using cryogenic air separation units.
Historically, the typical cryogenic air separation unit used a double distillation column with a crude argon (or argon side arm) column to recover argon from air. A good example of this typical unit is disclosed in an article by Latimer, R. E., entitled "Distillation of Air", in Chemical Engineering Progress, 63 (2), 35-59 [1967]. A conventional unit of this type is shown in FIG. 1, which is discussed later in this disclosure.
However, this conventional process has some shortcomings. U.S. Pat. No. 4,670,031 discusses in detail these shortcomings and explains the problems which limit the amount of crude argon recovery with the above configuration. This can be easily explained with reference to FIG. 1. For a given production of oxygen and nitrogen products, the total boilup and hence the vapor flow in the bottom of section I of the low pressure column is nearly fixed. As this vapor travels up the low pressure column it is split between the feed to the crude argon column and the feed to the bottom of section II of the low pressure column. The gaseous feed to the top of section II of the low pressure column is derived by the near total vaporization of a portion of the crude liquid oxygen stream in the boiler/condenser located at the top of the crude argon column. The composition of this gaseous feed stream is typically 35-40% oxygen. A minimum amount of vapor is needed in section II of the low pressure column, namely the amount necessary for it to reach the introduction point of the gaseous feed to the top of section II without pinching in this section. Since the composition of the gaseous feed stream to the top of section II is essentially fixed, the maximum flow of vapor which can be sent to the crude argon column is also limited. This limits the argon which can be recovered from this process.
In order to increase argon recovery, it is desirable to increase the flow of vapor to the crude argon column. This implies that the vapor flow through section II of the low pressure column must be decreased (as total vapor flow from the bottom of the low pressure column is nearly fixed). One way to accomplish this would be to increase the oxygen content of the gaseous feed stream to the top of section II of the low pressure column because that would decrease the vapor flow requirement through this section of the low pressure column. However, since this gaseous feed stream is derived from the crude liquid oxygen, its composition is fixed within a narrow range as described above. Therefore, the suggested solution is not possible with the current designs and the argon recovery is thus limited.
U.S. Pat. No. 4,670,031 suggests a method to increase the argon recovery which partially overcomes the above discussed deficiency. This is achieved by the use of an additional boiler/condenser. This additional boiler/condenser allows the exchange of latent heats between an intermediate point of the crude argon column and a location in section II of the low pressure column. Thus a vapor stream is withdrawn from an intermediate height of the crude argon column and is condensed in this additional boiler/condenser and sent back as intermediate reflux to the crude argon column. The liquid to be vaporized in this boiler/condenser is withdrawn from the section II of the low pressure column and the heated fluid is sent back to the same location in the low pressure column. A boiler/condenser is also used at the top of the crude argon column to provide the reflux needed for the top section of this column. A portion of the crude liquid oxygen is vaporized in this top boiler/condenser analogous to the conventional process. The use of the additional boiler/condenser provides some of the vapor at a location in Section II where oxygen content in the vapor stream is higher than that in the crude liquid oxygen stream. This decreases the minimum vapor flow requirement of this section and thereby allows an increased vapor flow to the bottom of the crude argon column. This leads to an increase in argon recovery.
Even though the method suggested in the U.S. Pat. No. 4,670,031 leads to an increase in argon recovery, it is not totally effective. This is due to the fact that all the vapor feed to the crude argon column does not reach the top of this column and an increased liquid/vapor ratio is used in the bottom section of this column. Since argon is withdrawn from the top of the crude argon column and a certain liquid/vapor ratio is needed in the top section to achieve the desired crude argon purity, the relatively lower vapor flow in the top section (as compared to the bottom section) limits the argon recovery. It is desirable to have a scheme, which will produce an increased vapor flow in the top section of the crude argon column so that argon can be recovered in even greater quantities.
U.S. Pat. No. 4,822,395 teaches another method of argon recovery. In this method all the crude liquid oxygen from the bottom of the high pressure column is fed to the low pressure column. The liquid from the bottom of the low pressure column is reduced in pressure and boiled in the boiler/condenser located at the top of the crude argon column. The crude argon column overhead vapor is condensed in this boiler/condenser and provides reflux to this column. There are some disadvantages to this method. The liquid from the bottom of the low pressure column is nearly pure oxygen and since it condenses the crude argon overhead vapor, its pressure when boiled will be much lower than the low pressure column pressure. As a result, the oxygen gas recovered will be at a pressure which is significantly lower than that of the low pressure column and when oxygen is a desired product this represents a loss of energy. Furthermore, this arrangement requires that the low pressure column operates at a pressure which is significantly higher than the ambient pressure. If nitrogen is not a desired product or if it is not needed at a higher pressure, then this process will require excessive energy consumption. Another drawback of the suggested solution is that since crude argon overhead is only condensed against pure oxygen, the amount of vapor which can be fed to the crude argon column is limited by the amount of oxygen present in the air. In some cases, this can lead to lower argon recoveries.
To generate an ultra high purity (&gt;99.5%) oxygen product, U.S. Pat. No. 4,615,716 proposes an oxygen recycle to increase the reboil vapor rate in the low pressure column bottom section. A vapor draw is taken from the low pressure column at the same location as the feed to the crude argon column. The stream is compressed and sent to an auxiliary reboiler for the low pressure column to generate additional vapor flow in the bottom section. The stream condenses in the reboiler and is sent back to the low pressure column at the same location as the crude argon column return stream. Although this method increases the reboil vapor rate, it does not allow for additional vapor to be sent to the crude argon column. As a result, additional argon recovery is limited.
U.S. Pat. No. 4,832,719 boils some liquid nitrogen from the top of the high pressure column against an argon-rich vapor stream from the crude argon column. In this scheme, argon recovery is not increased. This is because the boilup in the bottom of the low pressure column is not increased and therefore additional vapor flow to the crude argon column is not available. In this patent, a medium pressure nitrogen is coproduced as the added benefit. This patent recognizes that in certain cases, the amount of liquid nitrogen reflux needed by the low pressure column is less than that produced by the high pressure column. This difference in the liquid nitrogen can be boiled by an argon-rich vapor stream from the crude argon column to coproduce medium pressure gaseous nitrogen product.
U.S. Pat. No. 4,575,388 suggests an argon heat pump to increase argon recovery. A portion of the crude argon column overhead is warmed, compressed, and then condensed by boiling a liquid oxygen stream from the bottom of the low pressure column. The condensed argon is then returned to the crude argon column as overhead reflux. Although this invention effectively increases the boilup at the bottom of the low pressure column, which allows an increased feed rate to the crude argon column, the use of argon as a heat pump fluid is not desirable. This is in view of the fact that argon is such a highly valued product (as compared to nitrogen and oxygen) and any seal loss in the compressor can bring on significant penalty. On the other hand, the cost of a compressor package with negligible seal losses can be very high. Consequently, it is desirable to avoid using argon as the heat pump fluid.
Another method for increasing the argon recovery is through the use of the conventional low pressure nitrogen (LPGAN) heat pump. In this well-known method, the heat pump fluid is taken from the low pressure nitrogen product stream such as at the warmed outlet of the main exchangers. The stream gets compressed up to the appropriate pressure as determined by the high pressure column pressure. The stream is then cooled and sent to the high pressure column where it enters the boiler/condenser and condenses by boiling up the bottoms of the low pressure column. The condensed nitrogen stream is then sent to the low pressure column as additional overhead reflux. The limitation behind this heat pump, however, is that the compression requirement for the heat pump fluid is very high. As a result, this heat pumping strategy is very energy intensive and consequently economically unattractive.
Finally, another process teaching a method to improve argon recovery is taught in U.S. Pat. No. 5,114,449. This prior art process is shown in FIG. 2 which is also discussed later in this disclosure. In this process, all the crude liquid O.sub.2 from the bottom of the high pressure column is fed to the low pressure column. The vapor at the top of the crude argon column is now condensed by heat exchange with a liquid stream in the low pressure column. This heat exchange place is located between the crude liquid oxygen feed location and the withdrawal point of the argon-rich vapor stream which is the feed stream for the crude argon column. This thermal linkage between the crude argon and the low pressure columns leads to enhanced argon recovery when compared to the process shown in FIG. 1 and the one taught in U.S. Pat. No. 4,670,031. However, in certain instances, this enhanced argon recovery is still not sufficient to meet the increased demand of argon and it is desirable to envision methods which would further increase the argon recovery.
Clearly then, there is a need for a process which does not have the above-mentioned limitations and can produce argon with greater recoveries.